[coyoteX]

ultimx, This is the stock tire size for the HF. As for the VAC gauge, it's not constant pressure but the highest vacuum under any load condition that matters. You will see that conditions constantly change. On long hills I try to keep at least 2" of vacuum. On reasonably level terrain, I try to average 8" or better. Remember, I'm at 6000-7000 ft. elevation most of the time.
slownungly, What photos I have of the diffuser are in the Tadpole album. Basically, it's a 3 sided tapered box, that attaches at the rear, of the rear suspension, near the control arm pivots and at the bottom of the bumper cover. The front tire spats are more important than the rear ones. The leading edge of the front tires create as much drag as the mirrors do. Turbulence from everything else going on under the car minimize the need for the rear tire spats. Look under all newer cars at the body protrusions in front of the tires to get the idea. The side view of the car with my foot in the picture is the best profile shot. Note the thin flaps ahead of the front tires. Those create turbulence in front of the tire. I removed the ones you see in front of the back tires because they were to large and not really needed anyway. I also removed the small fins on the rear corners of the trunk lid. On new cars, the tire spats have a curb shape and are very short. You want to create a concentrated area of turbulence that will flow at the tire, so that it does not attach to the tire. Turbulence will set you free. Remember the divots on the cover of a golf ball.